Method and apparatus for digesting cellulosic material



June 21, 1938. D EY METHQD AND APPARATUS FOR DIGESTING CELLULOSICMATERIAL Filed May 11, 1954 Patented June 21, 1938 PATENT OFFICE METHODAND APPARATUS FOR DIGES TING CELLULOSIC MATERIAL Francis J. Dooley,Arlington, Mass. Application May 11, 1934, Serial No. 725,139

1 Claim.

This invention relates to the manufacture of wood pulp for paper and hasparticular reference to cooking or digesting processes and apparatuswherein the cooking or digesting liquor gives off a gas during thecooking process.

In the manufacture of sulphite pulp, for instance, the cellulosicmaterial as wood chips or blocks are treated in a digester with asulphite liquor or acid resulting from the reaction of sulphur dioxidegas with limestone and water. The liquor in contact with the wood chipsor other cellulosic material in the digester is heated and is maintainedin heated condition during the continuance of the cooking process, whichmay be upwards of eight hours. 'Due to-the elevated temperature of theliquor, sulphur dioxide gas is liberated therefrom. While the liberationof suificient gas to maintain the contents of the digesterunder'suitable elevated pressure, as from seventy to eighty pounds persquare inch, is desirable,' gas is liberated from the liquor in excessof that necessary to maintain the pressure and the excess gas heretoforehas been conducted away from the digester. The loss of gas from thedigesting liquor results in the weakening of the liquor and consequentlyprolongs the cooking time. Hence an object of the present invention isin the recovery of the usually wasted gas and its re-introduction intothe cooking liquor so as to enrich the liquor and restore it toapproximately its original strength thereby not only to' save thenormally wasted gas but also to reduce the cooking time.

A pulp mill is usually provided with a battery of digesters which arecharged and started in operation usually one at a time so that the firstdigester may be well advanced in operation by the time the next digesteris ready for operation. A further object is the'provision of a batteryof digesterswherein the gas evolved from the liquor in one digester isconserved by being discharged into and absorbed by the liquor in anotherdigester.

In accordance with the present invention the liquor in the digester isheated by circulating the liquor from the digester through a heatexchanger and back into the digester, the circulation preferably beingcontinuous during the cooking process. The heat exchanger is suppliedwith heat usually by steam at suitable pressure to maintain the desiredtemperature and pressure within the digester. The loss of gas from thecooking liquor and the digester is particularly serious with such anindirect liquor' heating system. The loss of gas from the liquor causesa liberation of the lime content of the liquor and the lime precipitatesor collects in greatest quantity on the heating surfaces or in the tubesof the heat exchanger. The lime accumulation impedesv the circulation ofliquor through the liquor passages of the heat exchanger and thetransfer of heat from the steam to the liquor and necessitates continualcleaning of the heat exchanger and repairs thereto. Hence a furtherobject of the present invention consists in maintaining the liquor inthe digester at approximately its normal strength at least in respect toits gas content so as .to prevent the deposition or precipitation of thelime or other solid matter in the heat exchanger and thereby to improvethe operation of the system.

A further object of the invention consists in conducting the gas that isliberated from the liquor within the digester into the circulatory pathof the liquor external of the digester and to cause it to be entrainedand absorbed by the liquor and reintroduced into the digester thereby tomaintain the normal strength of liquor and to prevent the precipitationor deposition of lime or othersolid matter. v

The circulation of the liquor between the digester and the heatexchanger is accomplished by a suitable pump and a further object of theinvention is to draw the gas that is liberated from the liquor withinthe digesterinto the inlet or suction piping of the pump by the actionof the pump. Also in.-accordance with this invention the gas from onedigester may be directed into the inlet piping of the circulating pumpof the same digester or into some other digester.

Fig. 1 is a diagrammatic view partly in section rangement 'forcirculating the digesting liquor and for introducing the gas thereinto.

Fig. 1 illustrates a digester system including, as a part thereof, thedigesters l0 and I2 of more or less common form having a charging neck Mat the top the opening in which can be closed by a cover I6 and havingan outlet l8, or the equivalent, at the bottom, controlled by. a valve20 through which pipe the contentsof the digester can be discharged whenthe cellulosic material has been sufiiciently cooked. The digesters arecharged with a digesting liquor as acid by a pump 22 having an inletpipe 24 in communication with a suitable source of acid and dischargingacid through the pipe 26 into an acid main 28, which the chips by theliquor.

main extends along the line of the series of digesters. Acid isconducted from the main through a pipe 30 controlled by a valve 32 intothe pipe l6 and thence into the bottom of the digester in accordancewith my Patent No. 1,338,496, dated April 27, 1920. The digester isadapted to be filled initially with acid to some suitably high level a-asomewhat under the neck I4 and above the top of the collection of chipsor other cellulosic material therein. The liquor in' the digester iscirculated through a heat exchanger 34 where it is heated and then isreturned to the interior of the digester. The heat exchanger may be ofany suitable type and as here shown comprises a shell or drum 36 havinginternal heads 38 and 46 between which tubes 42 extend. Liquor is drawnfrom the upper portion of the digester through a perforated or-screenedinlet pipe 44 and conducted to the upper part of the heat exchangerwhere the liquor flows through the tubes 42 and out of the bottom of thedigester into the inlet piping 46 of a suitable circulating pump 46which can'be of the centrifugal type. The liquor is discharged-from thepump into a pipe 50 which communicates with the piping l8 of thedigester and returns the heated liquor to the interior of the digesterat the bottom thereof. A heating medium is supplied to the interior oi!the heat exchanger around the tubes 42 through the pipes 52 and 54. Theheating medium can be live steam introduced through the pipe 54 anddischarged through the pi'ie 52 or vice versa. Because of thecirculation of the liquor of the digester through the heat exchanger theliquor becomes hot which is essential for the cooking or he digesting ofthe cellulosic material in the digester. My invention is particularlyconcerned with a digesting liquor known as sulphite liquor or acidformed by the reaction between sulphur dioxide gas, lime, usually in theform of limestone, and water. Due to the elevated temperature of theliquor, however, sulphur dioxide gas is liberated therefrom and collectsin the upper part of the digester above the liquor level. Someliberation of gas from the liquor is desirable since its accumulation inthe digester builds up a pressure which is beneficial in causing thepenetration of During the cooking process, however, much more gas isliberated from the liquor than is required to maintain the properpressure of from seventy to eighty pounds within the digester and it isnecessary to relieve the digester of the excess gas in order to keep thepressure within the digester to a safe value. Heretofore, this liberatedgas has been conducted away from the digesters. The escape of gas from,the liquor weakens the liquor and thus lengthens the time required forthe completionof the cooking process. The loss of gas also liberateslime from the sulphite liquor, and the lime precipitates. Theprecipitation occurs mainly within the tubes 42 of the heat exchangerwhere the heat is the greatest. This precipitate or deposit of limeadheres strongly to the interior of the tubes and collects and-opposesthe flow of liquor therethrough. The deposit also insulates the liquorfrom the tube surfaces so that the heat exchange is less effective. As aconsequence it is necessary to remove the lime deposits periodically bymechanical means. The lime deposits also cause pitting and othertroubles and frequent repairs to and replacement of parts of the heatexchanger are necessary.

In accordance with the present invention the sulphur dioxide-gas that isliberated from the liquor in the digester and would ordinarily bedischarged to waste is caused to be absorbed by the digester liquorwhereby to preserve the norinal strength of the liquor and also toprevent the end to the inlet piping 46 of the liquor circulating pump46. Thus the gas liberated from the liquor is caused to be entrainedinto the circulating liquor by the suction action of the pump and to bere-absorbed by the liquor and re-introduced into the bottom of thedigester, thereby preserving the normal strength of the liquor thereiniThe pipe 56 may be provided with a valve controlled branch 58 forinitially blowing oil? the air in the digester at the start of theheating operation.

Sulphur dioxide gas is soluble in water or the digesting acid and henceis readily absorbed by the acid when mixedtherewith in the acidcirculating system. The gas is under the pressure of the digester andthe liquid in the suction piping is under a lower pressure due to thepump suction and hence there is a positive pressure acting on the gas ina direction to causeit to flow into the circulating system. Since theacid is maintained at its original strength by restoring to the acid gasliberated therefrom, except as the strength may be weakened somewhat bythe reaction of the acid on the cellulosic material, the precipitationof lime from the acid is greatly reduced if not entirely eliminated andhence the cooking time is reduced, the acid is heated more rapidly inthe heat exchanger, less steam is consumed therein, and less cleaning ofand repairs to the heat exchanger are required.

While the system has been described primarily in connection with thedigester Hi, the digester l2, and all digesters, of the system are orcan be provided with the some equipment for the performance of theinvention.

A battery of dlgesters are usually charged with wood and acid andstarted in operation one at a time. It takes a matter of an hour or soto condition a digester for operation so that the first charged digestermay be well under-way when the second is ready for operation. I! desiredthe gas liberated from the liquor in the first digester can be passedinto the liquor oi. the second digester and so on through the series ofdigesters.

As is here shown by dotted lines in Fig. 1 the digester Hi can have agas pipe 60 leading there- .from into the suction piping 46 of thesecond digester l2 so that the gas from the digester I0 is passed intotheliquor or the digester l2 to enrich it. The gas in the digester l2can be passed ment of the heating and circulating system for the liquorin the digester. As here shown, the digester Illa. is provided with aheat exchanger 34a and liquor from the digester is withdrawn through asuction pipe 44a and through a pump The main dii-- trated in this figurethe gas evolved from the liquori'n the digester is passed through thepipe 56a and drawn into the suction pipe a by the suction action of thepump 48a. so that the liquor is enriched with its gas prior to itspassage through the heat exchanger.

I claim:

A digester system for cellulosic material comprising a battery ofdigesters adapted to receive the cellulosic materialand charges oil gasevolving liquor, heat exchangers associated with said digesters, meansfor circulating the liquor of a digester through the associated heatexchanger, and means for introducing the gas evolved from the liquor inonedigester into the circulating liquor of another digester externallythereof.

FRANCIS J. DOOLEY.

